1. Technical Field
This invention relates to computing systems, and more particularly, maintaining coherency among multiple cache memories.
2. Description of the Related Art
Computing systems may include one or more systems-on-a-chip (SoC), which may integrate a number of different functions, such as, graphics processing, onto a single integrated circuit. With numerous functions included in a single integrated circuit, chip count may be kept low in mobile computing systems, such as tablets, for example, which may result in reduced assembly costs, and a smaller form factor for such mobile computing systems.
To implement the desired functions on an SoC, one or more processors may be employed. Each processor may include a memory system with multiple levels of caches for providing low latency access to program instructions and operands. With multiple processors accessing multiple caches as well as main memory, the issue of cache coherency may arise. For example, a given data producer, such as, e.g., one of processors, may write a copy of data in a cache, but the update to main memory's copy of the data may be delayed. In write-through caches, a write operation may be dispatched to memory in response to the write to the cache line, but the write is delayed in time. In a writeback cache, writes are made in the cache and not reflected in memory until the updated cache block is replaced in the cache (and is written back to main memory in response to the replacement).
Because the updates have not been made to main memory at the time the updates are made in cache, a given data consumer, such as, e.g., another processor, may read the copy of data in main memory and obtain “stale” data (data that has not yet been updated). A cached copy in a cache other than the one to which a data producer is coupled can also have stale data. Additionally, if multiple data producers are writing the same memory locations, different data consumers could observe the writes in different orders.
Cache coherence solves these problems by ensuring that various copies of the same data (from the same memory location) can be maintained while avoiding “stale data”, and by establishing a “global” order of reads/writes to the memory locations by different producers/consumers. If a read follows a write in the global order, the data read reflects the write. Typically, caches will track a state of their copies according to the coherence scheme. For example, the popular Modified, Exclusive, Shared, Invalid (MESI) scheme includes a modified state (the copy is modified with respect to main memory and other copies); an exclusive state (the copy is the only copy other than main memory); a shared state (there may be one or more other copies besides the main memory copy); and the invalid state (the copy is not valid). The MOESI scheme adds an Owned state in which the cache is responsible for providing the data for a request (either by writing back to main memory before the data is provided to the requestor, or by directly providing the data to the requester), but there may be other copies in other caches. Maintaining cache coherency is increasingly challenging as various different types of memory requests referencing uncacheable and cacheable regions of the address space are processed by the processor(s).